The purpose of this invention is a linear angular position sensor with magnetoresistors. It is widely used in applications for measuring the orientation of a magnetic field, and more particularly for detection of the angular position of an object equipped with a magnetic field source. The sensor according to the invention has a measurement plane and it outputs a signal that is unequivocally related to the angle between the component of the field in this plane and a reference direction also located in this plane. The angular position of any object (part, motor, rotor, antenna, control button, etc.) may be identified by a sensor according to the invention.
Devices for detecting the orientation of a magnetic field using magnetoresistive materials based on ferromagnetic transition metals such as nickel, iron, cobalt or their alloys, are known. These materials in the form of bars can be used to make devices that output approximately sinusoidal signals as a function of the orientation of the applied field. These devices have the disadvantage that they require electronic processing means in order to obtain a measurement signal that is a linear function of the angle.
Sensors are also known that use stacks of layers leading to a giant magnetoresistance (GMR) effect as the magnetoresistive material. These stacks are usually composed of two thin magnetic layers separated by a thin non-magnetic and electrically conducting layer. Structures of this type, also called spin valves, are described in U.S. Pat. No. 4,949,039. A device for detecting the angular position of an object using this type of giant magnetoresistance effect is described in U.S. Pat. No. 5,650,721 and in the article by W. CLEMENS et al. entitled xe2x80x9cContactless potentiometer based on giant magnetoresistance sensorsxe2x80x9d, published in the Jour. Appl. Phys. 81, (8), Apr. 15, 1997, pp. 4310-4312. This technique uses one spin valve magnetoresistor, or possibly two laid out perpendicularly. The signal output by each magnetoresistor is approximately sinusoidal depending on the angle of the field with respect to a reference direction. But is not rigorously sinusoidal due to effects related to demagnetizing fields. Furthermore, there is a rotational hysteresis of 2xc2x0 that limits the resolution of the instrument. Furthermore, it is impossible to process fields outside the range 4.4 kA/m and 27.2 kA/m.
Other known sensors use materials with giant magnetoresistance effect with isotropic response. Remember that an isotropic magnetoresistive material is a material for which the resistance varies as a function of the field amplitude, but does not vary as a function of the field direction in the plane; the response is the same at all angles.
An angle detector composed of two magnetoresistors installed at 90xc2x0 to each other in a half-bridge cannot give a response that depends on the angle formed by the field and a reference direction in the plane, unless the response of each of these magnetoresistors in the field is different so that an unbalance can be created in the bridge.
In the case of an isotropic giant effect material, the two magnetoresistors for which the dimensions are defined by etching usually have the same isotropic response and there is no angle detection effect. One solution for making an angle detector with an isotropic giant effect material using two magnetoresistors at 90xc2x0 and a half bridge was given in patent DE 195 32674. This document proposes placing magnetic flux guides around the magnetoresistors, the function of which is different depending on which of the magnetoresistors is being considered, so that the response of the two magnetoresistors is different. For example, if the applied magnetic field is parallel to one of the magnetoresistors, it will be perpendicular to the other and the flux guides capture all flux lines created by the field and which should have entered the first magnetoresistor. Therefore, the first magnetoresistor does not see the field and its change in resistance is zero. But conversely, these guides concentrate the entire flux in the second magnetoresistor, and consequently the field and the resistance of the second magnetoresistor will reduce.
Obviously, the bridge is unbalanced and the sensor outputs a response.
This invention proposes to avoid these flux guides, while making sure that the two magnetoresistors give different responses.
This invention proposes to make anisotropic shaped magnetoresistors to confer an anisotropic response to them as a function of the field (whereas the material is intrinsically isotropic). This anisotropy shape may be achieved by etching with appropriate dimensions. Furthermore, the invention consists of a particular assembly (a bridge) in which the read signal is taken off at the midpoint of the bridge. Surprisingly, the signal obtained is linear over a very wide angular range (better than 5% over 90xc2x0). Consequently, there is no longer a need to use signal processing electronics to linearize the read signal. Furthermore, the hysteresis has disappeared. Finally, in an assembly according to the invention, there is no longer any need to use polarization of magnetoresistors.
More precisely, the purpose of this invention is an angular sensor comprising at least two magnetoresistors and a material with giant magnetoresistive effect and with isotropic response, these magnetoresistors being placed perpendicularly, characterized by the fact that the two magnetoresistors have anisotropy shape such that their response in a magnetic field is anisotropic, the two magnetoresistors being connected in series and forming a bridge with two ends and one midpoint, the power supply means being connected at the ends of the bridge and means of measuring the unbalance of the bridge being connected at the midpoint of the bridge.
In one advantageous embodiment, the invention combines two bridges of this type to form a Wheatstone bridge with four magnetoresistors.
The linearity of the sensor may be further improved by the addition of a resistance made of a magnetosensitive material mounted either in series with the input of the bridge, or in parallel on its output.
Advantageously, the magnetoresistors are made from a stack of layers based on FeNi/Ag. Obviously, other stacks may be used and particularly stacks based on Fe/Cr or Fe/Cu or NiFe/Cu.